With an increase in capacity of information processing in recent years, various information recording technologies have been developed. In particular, the surface recording density of an HDD using magnetic recording technology is continuously increasing at an annual rate of approximately 60%. In recent years, an information recording capacity exceeding 320 gigabytes per platter has been desired for a magnetic recording medium with a 2.5-inch diameter for use in an HDD or the like. To fulfill such demands, an information recording density exceeding 500 gigabits per square inch is desired to be achieved.
Important factors for increasing recording density of the perpendicular magnetic disk include, for example, an improvement in TPI (Tracks per Inch) by narrowing the track width, ensuring electromagnetic conversion characteristics, such as a Signal-to-Noise Ratio (SNR) and an overwrite (OW) characteristic at the time of improving BPI (Bits per Inch), and further ensuring heat fluctuation resistance with recording bits decreased due to the above. Among these, an increase in SNR in a high recording density condition is important.
In a magnetic layer of a granular structure, which goes mainstream in recent years, a nonmagnetic substance having an oxide as a main component is segregated around magnetic particles having a CoCrPt alloy growing in a columnar shape to form a grain boundary part. In this structure, since the magnetic particles are separated from each other, noise is reduced, and this is effective for a high SNR. An important factor to further increase the SNR is to increase crystal orientation. Co takes an hcp structure (a hexagonal close-packed crystal lattice structure), and a c-axis direction (an axial direction of a hexagonal column as a crystal lattice) serves as an axis of easy magnetization. Therefore, by orienting the c axis of each of more crystals in a more perpendicular direction, noise is reduced and signals become strong, which can cause an increase in SNR as a synergy effect.
When a film of a metal with an hcp structure is formed by sputtering, crystal orientation tends to be improved as the film thickness is thicker. Thus, to enhance crystal orientation of a granular magnetic layer from an initial growth stage, conventionally performed are processes of forming a film of a ground layer made of Ru, which is a metal with an hcp structure, and then forming a film of the granular magnetic layer on the ground layer. Furthermore, a crystalline preliminary ground layer (which is also called a seed layer) is provided under the Ru ground layer, and crystal orientation of the Ru ground layer is improved.
Patent Document 1 discloses a structure in which a soft magnetic film forming a backing layer has an amorphous structure, a ground film (which corresponds to a preliminary ground layer in the present application) is made of a NiW alloy, and an intermediate film (which corresponds to a ground layer in the present application) is made of a Ru alloy. According to Patent Document 1, it is described that, with the ground film being made of a NiW alloy and the intermediate film being made of a Ru alloy, recording and reproduction of information can be made with high productivity and high density.
However, as a result of increasing a layer above the soft magnetic layer, when the film thickness is increased, a distance between the magnetic layer and the soft magnetic layer (hereinafter referred to as a “film thickness of the intermediate layer”) is increased. With an increase of the film thickness of the intermediate layer, a distance between the soft magnetic layer and a head becomes longer, and the soft magnetic layer cannot sufficiently draw a write magnetic field outputted from the head. As a result, the overwrite characteristic (OW characteristic) of the medium is worsened, write signal quality is degraded and, in turn, read signal quality (SNR) is degraded.
To reduce degradation of the overwrite characteristic and the SNR, it has been studied that the soft magnetic layer is provided with the function of the preliminary ground layer to reduce the film thickness above the soft magnetic layer. In Patent Document 2, a technology is suggested in which an intermediate layer is made thinner by forming a seed layer (a preliminary ground layer) made of a soft magnetic material.